New Early Cambrian Chordates from Haikou, Kunming

Two new chordates, Cathaymyrus haikouensis Luo et Hu sp. nov. and Zhongxiniscus intermedius Luo et Hu gen. et sp. nov. are studied in this paper. Both display numerous S-shaped myomeres on their trunk. C. haikouensis shows a long and slim body similar to that of Cathaymyrus diadexus Shu et al., Zhongxiniscus approaches to Myllokunmingia and Haikouichthys in the dorsal fin, but differs in the myomeres. Zhongxiniscus may be the intermediate form between Cathaymyrus and Myllokunmingia and Haikouichthys.

Core value of the Chengjiang fauna:formation of the animal kingdom and the birth of basic human organs

Well known for its abundant and extraordinary soft-tissue fossils,the Chengjiang fauna has witnessed the main phase of the Cambrian explosion and the first great congress of the ancestors of nearly all major phyla of animals on Earth.The large-scale survey and exploration of the fauna by Chinese paleontologists,partly in collaboration with international scientists,over the past 30 years can be broadly divided into three stages.In the first decade since 1984(19841994),a large number of invertebrates,including basal animals and the protostomes of the early animal tree were discovered,but the subkingdom Deuterostomia was completely unknown.Then in the second decade(19952005),the great discoveries of various deuterostomes led to the construction of the basic framework of the Deuterostomia and then to the formation of the tripartite phylogenetic trees of early animals(TPTEA,including basal animals,protostomes and deuterostomes),for the first time.In the third decade(after 2005),the academic community began to think about the internal relationships between the formation of TPTEA and the multiepisode Cambrian explosion,leading to the new hypothesis of the three-episode Cambrian explosion.The Chengjiang fauna is important for deciphering the fauna structure,paleoecological environment,and so on.However,its core academic values mainly rest on two aspects.Firstly,The Chengjiang fauna,as the main phase witness of the Cambrian explosion,has created a nearly complete phylogenetic framework of the TPTEA on Earth for the first time.The three-phase radiation hypothesis reveals the essential connotation of the Cambrian explosion:a step-wise divergent evolution of animals,from basal to highly advanced groups,lasting about 40 million years.In the first phase,it gave birth to a bulk of basal animals(including some now extinct“animal”groups)in the latest Ediacaran,probably including some pioneer protostomes.The second phase took place in the first epoch of the Cambrian period(Terreneuvian),giving rise to the main invertebrate protostomes with a persistent prosperity of basal animals.The third phase proceeded in Cambrian Epoch 2(represented by the Chengjiang fauna),which not only maintained the prosperity of basal animals and protostomes,but also,more importantly,gave birth to all the main phyla of the subkingdom Deuterostomia.Thus,the rudimental framework of the whole TPTEA has been shaped,with the termination of the major innovation events of the Cambrian explosion.Here,we discussed the evolutionary properties of Ediacaran biota,small shelly fossils and the Chengjiang fauna in the Cambrian explosion with emphasis on the biological properties of several important animal groups.The order Myllokunmingiida is the only known oldest vertebrate,while Yunnanozoon and Haikouella are neither vertebrates nor stem-group chordates but a special group of basal deuterostomes;Cheungkongella is a credible ancestor of the urochordate and it supports the classical hypothesis on the origin of the urochordates;and the gill slits were first invented in the members of the phylum Vetulicolia to provide key information on the origin of the deuterostomes.The second core value of the Chengjiang fauna is of profound humanistic and philosophical significance:the discoveries of the‘first gill openings’,‘first brain’,‘first vertebrae’and‘first heart’provide the pivotal evidence for solving the unsolved mystery of the origin of the main basic human organs as described in Darwin’s“The Descent of Man”.In addition,the morphological and anatomical information of the Chengjiang fauna can provide important clues for a better understanding of most components of Ediacaran and Cambrian metazoans.

Amphioxus Gli knockout disrupts the development of left–right asymmetry but has limited impact on neural patterning

The Gli transcription factors are the primary mediators of Hedgehog(Hh)signaling.Vertebrate genomes contain multiple Gli paralogues with different functions downstream of Hh signal receipt,in part explaining the complexity of cellular responses to Hh that allow concentration-dependent target gene activation.Amphioxus is a chordate that split from the vertebrate lineage early in the evolution of chordates,before the genome duplications that occurred in early vertebrate evolution.It has a single Gli gene whose transcripts can be alternately spliced to yield two protein isoforms called GliS and GliL.We generated two knockout mutations in amphioxus Gli,one that affects the whole gene and a second that only affects GliL.Both knockouts showed major morphological and molecular defects in the development of left–right asymmetry,a phenotype that is similar but not identical to that previously found in Hh mutants.Hh signaling also patterns the amphioxus neural tube.Here,however,knockout of GliL showed no identifiable phenotype,while knockout of the full gene showed only small changes to the expression of one gene family,Olig.Other genes that were prominently affected by Hh knockout were not altered in expression in either knockout.Reasons for the differences between Hh and Gli knockouts in the pharynx and neural tube are discussed in the context of the likely different functions of amphioxus Gli isoforms.

Revaluation of deuterostome phylogeny and evolutionary relationships among chordate subphyla using mitogenome data

The traditional knowledge in textbooks indicated that cephalochordates were the closest relatives to vertebrates among all extant organisms. However, this opinion was challenged by several recent phylogenetic studies using hundreds of nuclear genes. The researchers suggested that urochordates, but not cephalochordates, should be the closest living relatives to vertebrates. In the present study, by using data generated from hundreds of mtDNA sequences, we revalue the deuterostome phylogeny in terms of whole mitochondrial genomes （mitogenomes）. Our results firmly demonstrate that each of extant deuterostome phyla and chordate subphyla is monophyletic. But the results present several alternative phylogenetic trees depending on different sequence datasets used in the analysis. Although no clear phylogenetic relationships are obtained, those trees indicate that the ancient common ancestor diversified rapidly soon after their appearance in the early Cambrian and generated all major deuterostome lineages during a short historical period, which is consistent with ＂Cambrian explosion＂ revealed by paleontologists. It was the 520-million-year＇s evolution that obscured the phylogenetic relationships of extant deuterostomes. Thus, we conclude that an integrative analysis approach rather than simply using more DNA sequences should be employed to address the distant evolutionary relationship.

Diversification of amphioxus and vertebrate Cerberus protein function in modulating Nodal,BMP and Wnt signals

Cerberus is a multifunctional antagonist implicated in embryonic patterning through modulation of Nodal,BMP and Wnt signals.Although its function is largely conserved in chordates,certain activities have diverged,even among vertebrates.Moreover,the antagonistic action of Cerberus from the basal chordate amphioxus toward Nodal,BMP and Wnt signals remains elusive.Here,we compared the activity of amphioxus and Xenopus Cerberus proteins using cross-species assays.We found that amphioxus and Xenopus Cerberus proteins display similar activities in antagonizing Nodal-induced events,but they exhibit both shared and distinct activities in modulating BMP and Wnt signals.Amphioxus Cerberus has reduced neuralizing activity that is dependent on inhibition of BMP signaling,and it modulates the signals of a restricted subset of Wnt proteins.Furthermore,we revealed that Xenopus Cerberus interacts with Wnt4 and Wntl 1 to activate canonical Wnt signaling,whereas amphioxus Cerberus lacks this activity.These differences may be correlated with the divergence in the N-terminal region of Cerberus proteins between amphioxus and Xenopus.Our results indicate that chordate Cerberus proteins have evolved sub-functionalities that depend not only on their concentrations,but also on the properties of BMP and Wnt signals.This may account for their evolutionary distinct functions in different patterning processes.